There are many apps for sending text messages from the iPad. I've used Google Voice and Talkatone, both of which are free. There are others, but I've not used any, I am sure there is a bunch in the App Store.
They don't work for MMS, however. The only app I've seen for using MMS (to send pictures or videos) is iTim, which is a paid app. I've not used it, so I can't verify how well it works. There may be others, but again, I don't know.
Hopefully, these will give you someplace to start. And, maybe someone can chip in with their recommendations. I used Talkatone and Google Voice with the iPad all the time (before I got my smart phone). I never had any trouble with sending texts to any mobile phone. I didn't send MMS messages, though, so there's that.
Hope some of this helps.
Marilyn